Piston for hydraulic translating unit

ABSTRACT

A piston for use in an axial and/or radial piston hydraulic translating unit, comprising a hollow skirt portion having inner surfaces defining a cavity and filled with a lightweight material, the material retained in place by discontinuous irregularities in the surface of the hollow skirt which mechanically interlock the filler material within the piston.

United States Patent Hein et a1.

[ 1 Dec. 26, 1972 [541 PISTON FOR HYDRAULIC TRANSLATING UNIT [72] Inventors: Allyn J. Hein; Gilbert Tribley, both of Joliet, Ill.

[73] Assignee: Caterpillar Tractor Co., Peoria, 111.

[22] Filed: Feb. 16, 1971 [21] App]. No.: 115,639

[52] US. Cl ..92/248 [51] Int. Cl. ..Fl6j l/00 [58] Field of Search ..92/172, 248, 249; 91/499 [56] References Cited UNITED STATES PATENTS 1,204,889 11/1916 Lilly "92/248 1,433,478 10/1922 Rogatchoff ..92/172 2,741,518 4/1956 Leman ..92/249 X 3,080,854 3/1963 Wiggermann ..92/172 X 3,187,644 6/1965 Ricketts ..92/248 FOREIGN PATENTS OR APPLICATIONS 922,038 H1955 Germany ..92/249 Primary Examiner-Edgar W. Geoghegan Assistant Examiner-4min C. Cohen Attorney-Fryer, Tjensvold, Feix, Phillips & Lempio [5 7] ABSTRACT A piston for use in an axial and/or radial piston hydraulic translating unit, comprising a hollow skirt portion having inner surfaces defining a cavity and filled with a lightweight material, the material retained in place by discontinuous irregularities in the surface of the hollow skirt which mechanically interlock the filler material within the piston.

3 Claims, 3 Drawing Figures PATENTED 3.707.113

INVENTORS ALLYN J. HEIN GILBERT TRIBLEY BY 9x 3L any P E4414 PISTON FOR HYDRAULIC TRANSLATING UNIT BACKGROUND OF THE INVENTION Certain hydraulic translating units utilize orbiting pistons to convert mechanical power into hydraulic power and vice versa. These translating units include any one of several well'known means to transform rotational movements into reciprocating movements and vice versa as the pistons orbit around a rotational axis.

Conventionally, the weight of the orbiting and reciprocating pistons of a hydraulic translating unit is reduced by hollowing out the elongate, cylindrical skirt of the pistons. This, however, increases the dead volume of fluid that must be moved within a translating unit and has the effect of decreasing the efficiency of those units. Consequently, it is desirable to refill the hollowed out skirt with a lighter weight material possessing a sufficiently high bulk modulus to resist compression under the high pressures encountered.

Such filler material must be retained in some manner so that it will not be displaced during operation of the translating units, thereby possibly causing severe damage to such units.

SUMMARY AND OBJECTS OF THIS INVENTION This invention relates to a lightweight piston wherein material is removed by hollowing out the elongate, cylindrical skirt and is replaced by a filler material to reduce the total weight thereof. Means have not been perfected wherein the filler material is positively retained in place by a mechanical interlock established between discontinuous irregularities on the surface of the hollowed out skirt of the piston and the filler material.

The object of this invention, therefore, is to provide an effective and inexpensive means of retaining a lightweight filler material within the hollow skirt of hydraulic translating unit pistons.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects of this invention will become apparent from the following description and accompanying drawings wherein:

FIG. 1 is a longitudinal cross-sectional view of a piston embodying one means for retention of the filler material and illustrating a means to provide an axial center bore in the filler material;

FIG. 2 is a cross-sectional view of the piston of FIG. 1 with filler material in place; and

FIG. 3 is a central cross-sectional view of a piston similar to FIG. 1 incorporating the embodiment of a second retention means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to thedrawing, a piston comprises generally a hollow, elongate, cylindrical skirt section or member 12 and a ball or base section 14 having surfaces defining a cavity 13. The ball or base section 14 cooperates with a conventional piston shoe (not shown) for assembly into a translating unit. The ball section 14 is provided with a drilled passage 16 to direct lubrication to a load bearing area between the ball section 14 and its cooperating piston shoe in a conventional manner. Surface 18 of the hollow skirt 12 is provided with discontinuous irregularities which, in the specific embodiment shown in FIG. 1, are in the form of recesses 20. The recesses are either symmetrically or randomly located on the inner wall and are formed by any suitable method such as grinding or punching.

With the above described arrangement, as the filler material is molded or introduced by any other suitable method into the cavity 13, the filler material will fill the recesses 20-and thereby form a mechanical interlock with the piston. The mechanical interlock is effective to prevent movement, either rotational or axial, or the filler material relative to the piston during operation of the translating unit. Any lightweight material (metal, epoxy, resin, etc.) having a sufficiently high bulk modulus to resist compression under high pressure may be used. In one application, an epoxy (Stycast Epoxy No. 1090 from Emerson & Cuming, Inc., Canton, Mass.) has been used as a filler material. The epoxy is heated until its plasticity is such that it can easily be poured into the hollow skirt of the piston. The epoxy is then cured for 24 hours at 200 Fahrenheit to assure polymerization and solidification and thereby attains the proper solid rigid properties to lock it against the skirtsurface l8 and within the recesses 20 for effective operation in the translating units.

In the above application, the depth of the recesses is preferably at least 0.007 inches to counteract the effect of contraction of the epoxy material as it solidifies.

The above referenced epoxy material will contract approximately 0.0020.003 inches per inch of length or per inch of diameter as it cools, after being deposited in the hollow skirt of the piston, whereby it is apparent that the noted epoxy material and recesses of about 0.007 inches can be safely used in pistons whose cavities 13 are in the order of 2 inches in diameter. The coefficient of expansion (thermal expansion) of the piston 10 material (steel) is 6.28 XlO'and for the above filler material is 9.93 XIO'. The difference in thermal expansion must also be considered when determining the required depth of the recesses.

The use of recesses having a depth greater than that specified above may be dictated in some instances by practical considerations such as cost or difficulty in forming the recesses, reductions of wall thickness of the pistons as related to mechanical strength thereof, substitution of a filler material having shrinkage or contraction characteristics different than that described hereinafter. In any event, however, the recesses 20 are of sufficient depth to secure the filler material within cavity 13 even after shrinkage that may occur subsequent to emplacement. In this regard, it will be apparent that the filler material is most advantageously poured into the cavity 13 while in the fluid or plastic state whereby it will flow into intimate contact with all of surface 18 and recesses 20.

FIG. 1 also illustrates a rod 22 (shown in phantom) placed in the bore 16 of the ball section 14, prior to pouring the filler material into the hollow skirt 12 to provide a lubrication passage through the tiller material. This will eliminate the need for drilling a passage in the filler material after it has solidified. The rod 22 should be coated with a Mold Release (Compound 1228 from Emerson & Cuming, Inc., for example) so that it can be easily removed after the filler material has solidified.

FIG. 2 illustrates a completed filled piston with a filler material 24 shown mechanically interlocked in the cavity formed by the hollow skirt [2. Also shown is a bore 26 formed by the removal of the rod 22 (FIG. 1) from the finished piston.

A second embodiment of the invention wherein the inner surface of the cavity 13 of piston 10 is randomly studded with protrusions 28, to form an irregular surface, is illustrated in FIG. 3. The protrusions 28 are formed, for example, by touching the inner surface of the skirt with a weld rod to form weld deposits thereon or by any other suitable method. The protrusions must extend inwardly from the inner surface of the skirt at least 0.007 inches for entrapment into the filler material a sufficient amount to compensate for contractionof the filler material, as well as' to counteract for differences in thermal expansion of the piston and filler material, as discussed earlier.

, Lightweight filled pistons in any of the above described embodiments virtually eliminate the possibility of the filler material loosening or becoming dislodged from the cavity during operation, and the chances of piston malfunction are thereby greatly reduced.

lclaim:

1. A piston of the type used in hydraulic translating units comprised of a generally hollow elongated cylindrical member, the inner surface of said hollow 

1. A piston of the type used in hydraulic translating units comprised of a generally hollow elongated cylindrical member, the inner surface of said hollow member defining a cavity within said piston, a plurality of randomly disposed, irregularly reentrant shaped protrusions of material affixed to said inner surface, a lightweight filler material substantially filling said cavity and enclosing said protrusions and inner surface to mechanically interlock said filler material within the piston.
 2. The piston of claim 1, wherein said lightweight filler material is an organic resinuous material.
 3. The piston of claim 1, wherein said protrusions of material are weld rod deposits. 